1. Field of the Invention
The present invention relates to a procedure for the continuous preparation of oxamide.
2. Discussion of the Background
Oxamide is an organic compound which is used in the synthesis of the widest variety of products ranging from esters, dialkylamides and hydrazides of oxalic acid, to copolyoxamides or copolymers of a bicarboxylic acid with the oxamide of an aliphatic or aromatic diamine; these compounds are described for example in Italian patent applications 20121 A/86, 20274 A/88 and 20276 A/88 filed by the present applicant.
The use of oxamide as a slow release fertilizer is just as well-known [Development in Plant and Soil Sci., 15, Fertilizer Manual, Ed. T. P. Higne; W. Junk Publishers (1985), page 279]; it is based on the slow release of nitrogen by the oxamide.
In spite of the affirmed and increasing use of oxamide, this compound is still not produced on a wide scale because of the relatively high costs and complicated synthesis methods. There are basically three of these methods: the Degussa process (GB 1,251,721), which is based on the oxidation of HCN with hydrogen peroxide, the Sagami process (DE 2034208), which is based on the use of NO.sub.2 instead of hydrogen peroxide, and the Hoechst process (DEO 230894, DE 2403120) which is based on the oxidation of HCN with oxygen and air.
Of the three above processes, the first two are rather expensive, both for the type of oxidant used and for the necessity of isolating cyanogen (N.tbd.C--C.tbd.N) as an intermediate product to be hydrolyzed in a subsequent step.
The Hoechst process on the other hand is considered the most accessible and economical of the three, both for the low cost of the oxidant used (oxygen and air), and for the synthesis procedure which consists of a single step; in addition all the products can be recycled, catalyst included.
The catalyst used is an aqueous solution of copper nitrate which is used in the presence of an aliphatic carboxylic acid (acetic acid). A subsequent patent (DE 2402354) emphasizes, in the economy of the process, the role of the oxidant (oxygen or air), which must be advantageously used in great excess with respect to the stoichiometric quantity.
The relative reaction scheme as defined by the authors (W. Riemenschneider, Chemtech., October 1976, pages 658-661) is specified below whereas the scheme of the Hoechst plant is specified in the same article.
2 Cu(NO.sub.3).sub.2 +4 HCN.fwdarw.(CN).sub.2 +2 CuCN+4 HNO.sub.3 PA1 2 CuCN+6 HNO.sub.3 .fwdarw.2 Cu(NO.sub.3).sub.2 +2 HCN+2 H.sub.2 O+2 NO.sub.2 PA1 2 NO.sub.2 +1/2 O.sub.2 +H.sub.2 O.fwdarw.2 HNO.sub.3 PA1 2 HCN+1/2 O.sub.2 .fwdarw.(CN).sub.2 +H.sub.2 O (CN).sub.2 +2 H.sub.2 O.fwdarw.H.sub.2 N--CO--CO--NH.sub.2 PA1 2 HCN+1/2 O.sub.2 +H.sub.2 O.fwdarw.H.sub.2 N--CO--CO--NH.sub.2 PA1 there is a heterogeneous distribution of the reagents; in fact the oxygen tends to be distributed preferentially in the upper part of the reactor whereas the hydrogen cyanide, owing to its solubility in the catalyst, tends to move both upwards and downwards with respect to the feeding point, PA1 oxygen is never present in the suspension taken from the thickening zone (F) of the reactor whereas the gases contained therein are basically composed of N.sub.2, NO, CO and CO.sub.2, PA1 the recycled catalytic solution always contains reasonable quantities of formamide and acetamide which are destroyed (by oxidation) when re-fed into the reactor as such a quantity does not increase with time.
The scheme of the reaction zone of the Hoechst process is shown in FIG. 1.
The Hoechst process, as described in the above patents and article, is carried out by continuously feeding in in the lower part of a tubular bubble reactor (T) containing the aqueous acid solution of the catalyst, HCN together with oxygen. The oxamide which is continuously formed precipitates in a crystalline form and is collected at the bottom of the reactor (F), which has a conical shape, in the so-called thickening zone, from which it is transferred as a suspension to a centrifuge, where the oxamide is separated from the catalytic solution which is recycled to the tubular bubble reactor (T); the latter solution is regenerated by adding copper nitrate and the quantity of water consumed according to the stoichiometry of the reaction. The thickening zone of the solid (F) is situated below the inlet of the oxygen and HCN.
In the upper part of the reactor there is an exchanger which cools the off-gases and condenses the vapours of water and acetic acid.
The excess oxygen, hydrogen cyanide and unconverted cyanogen, nitrogen oxides (N.sub.y O.sub.x), CO, CO.sub.2, N.sub.2, deriving from the secondary reactions, leave the head of the tubular reactor together with water and acetic acid.
The results of this process are still not satisfactory however from the point of view of production on an industrial scale because:
The presence of NO in the suspension continuously removed from the thickening zone (F) of the reactor, together with the lack of oxygen therein, indicates insufficient re-oxidation of this gas; in fact, in the oxidative process for the formation of oxamide, the nitrates can be reduced to products having a lower oxidation level (NO and NO.sub.2) which however, if there is a strong excess of oxygen, as is also recommended in German patent application DE 2402354, should be re-oxidized to nitric acid.
In addition, the lack of oxygen in the thickening zone (F) of the reactor favours the accumulation of reduced products (e.g. HNO.sub.2) which can attack the oxamide, oxidizing it exhaustively with the formation of gaseous products such as those specified above, in the suspension at the bottom; for example: EQU H.sub.2 N--CO--CO--NH.sub.2 +2HNO.sub.2 .fwdarw.2N.sub.2 +CO+CO.sub.2 +3H.sub.2 O
The carbon monoxide which is formed according to this reaction, as it is also flammable, increases the concentration of combustible products (hydrogen cyanide, acetic acid, cyanogen) in the gaseous phase and makes more critical the control of the explosivity of the gaseous mixture both in the head of the reactor and in the bubbling-bed.